In fresh attacks on symbols of state power, twin suicide bombs have exploded overnight (NZ time) near a government security compound in northern Syria and rockets struck the central bank in Damascus, killing nine people and wounding 100.

The regime and the opposition traded blame, accusing each other of dooming a United Nations plan to calm violence that has largely failed so far. The head of the UN observer mission acknowledged that his force cannot solve the country's crisis alone and urged both sides to stop fighting.

The attacks are the latest in a series of suicide bombings that started in December and have mostly targeted Syrian military and intelligence positions.

The regime routinely blames the opposition, which denies having a role or the capability to carry out such attacks. After other similar bombings, US officials suggested Al Qaeda militants may be joining the fray, and an Al Qaeda-inspired Islamist group has claimed responsibility for previous attacks in Syria.

The powerful blasts, which blew two craters in the ground and ripped the facade off a multistory building, came a day after Major General Robert Mood, the head of the observer mission, took up his post in Damascus.

"Ten, 30, 300 or 1000 observers will not solve all problems," he told reporters overnight (NZ time). "So everyone has to help us achieve this mission."

More than 9000 people have been killed in the 13-month crisis, according to the UN.

An April 12 cease-fire agreement has helped reduce violence, but fighting persists, and UN officials have singled out the Syrian regime as the main aggressor.

An advance team of 16 UN observers is on the ground to try to salvage the truce, which is part of a broader plan by special envoy Kofi Annan to launch talks between President Bashar Assad and his opponents. By mid-May, the team is to grow to 100, but UN officials have not said when a full 300-member contingent is to be deployed.

Last night's (NZ time) bombs went off in the northern city of Idlib, an opposition stronghold that government troops recaptured in a military offensive earlier this year. TV footage of the aftermath from the blasts showed torn flesh, burned-out cars, twisted debris and pavement stained with blood. The force of the explosions shattered windows and sent debris flying for hundreds of metres.

"Is this their freedom?" one man yelled at the cameras at one of the blast sites.

A distraught woman shouted: "What have we done to those people? What have women, children and the elderly done to them?"

The state-run news agency SANA said security forces and civilians were among those killed. State TV said that many of the nearly 100 wounded were civilians.

The bombers detonated their explosives near a military compound and near Idlib's Carlton Hotel, SANA said.

A local activist, who only gave his first name, Ibrahim, for fear of repercussions, said the two sites bombed in Idlib are several hundred meters apart and that the explosions went off within five minutes of each other after daybreak Monday.

There was no immediate claim of responsibility for the attacks. State media blamed "armed terrorists," a term it uses for rebels trying to topple the government. Activists claimed the regime was behind the bombings to discredit the opposition.

A statement by the Local Co-ordination Committees activist network called the series of suicide blasts "fabricated, staged explosions" and said "they can no longer fool anyone."

Two members of the UN observer team toured the site of the bombings, SANA said. Ibrahim said the observers have been staying at the Carlton, and a pro-government website reported that the hotel sustained some damage.

Analysts said it was doubtful the presence of UN observers would help improve the situation or halt such bombings.

"The UN is a political body, not an investigative body. The UN creates a political consensus among countries, but it's not a judge and jury about which side violated what agreement," said Jon Alterman, Middle East director at the Center for Strategic and International Studies in Washington.

An Al Qaeda-inspired Islamist group called the Al-Nusra Front to Protect the Levant claimed responsibility Monday for a suicide bombing in downtown Damascus that killed at least 10 people on Friday. The Associated Press could not verify the authenticity of Al-Nusra's statement, which was posted on a militant website.

Top US intelligence officials also have pointed to al-Qaida in Iraq as the likely culprit behind the previous bombings, raising the possibility that its fighters are infiltrating across the border to take advantage of the turmoil.

Al Qaeda's leader called for President Bashar Assad's ouster in February.

Earlier Monday, gunmen fired rocket-propelled grenades at the central bank and at a police patrol in the capital of Damascus, wounding four officers, SANA said.

The bank's governor, Adib Mayaleh, said the only damage to the bank was shattered windows.

He also denied reports that Syria is trying to sell gold reserves to raise money as international sanctions take their toll. French Foreign Minister Alain Juppe said earlier this month that sanctions have reduced Syria's foreign currency reserves by half, from an estimated US$17 billion (NZ$17.4b) at the start of the uprising.

The bank doesn't need to sell gold "as we have a big quantity of hard currency that can stand up to all those attacks," Mayaleh said.

As part of the cease-fire agreement, Syria's military was to have pulled tanks and troops off the streets, but it has instead continued to raid and attack opposition strongholds.

Near Damascus, amateur video posted Monday showed dozens of uniformed troops in helmets and body armor marching through a street in the suburb of Douma. A local activist, Mohammed Saeed, said the troops were carrying out arrests for a second day Monday.

In another suburb, Zamalka, activists said security forces tried to break a commercial strike by damaging shops that had been closed in solidarity with the protest.

In London, British Foreign Secretary William Hague warned that "there is a limit to the patience of the international community" with the regime's continued truce violations.

However, Western powers have limited options because Russia and China, Syria's allies, have shielded Assad from UN Security Council action.